Design Fink, Howe, King Post, Scissor, and Attic roof trusses. Calculate chord lengths, internal web members, apex heights, package counts, prices, and tension/compression forces.
Roof trusses achieve tremendous strength-to-weight ratios through rigid geometric triangulation:
Downward roof loads put top chords into pure compression. The bottom chord acts as a giant tension tie, eliminating horizontal thrust and transferring 100% of vertical reactions to exterior wall plates without needing interior support walls.
Truss joints are pressed under 50-ton hydraulic machinery with galvanized steel toothed plates (20-gauge). These plates lock lumber joints together with zero mechanical slippage, far exceeding hand-nailed stick framing.
Selecting the right truss depends on your clear-span length and interior ceiling requirements:
The undisputed benchmark for single-family homes. The W-web divides each top chord into two equal panels, perfectly balancing bending moments for spans from 20 to 40 feet.
Features an upward-sloping bottom chord (typically 2/12 to 3/12 shallower than exterior pitch) creating dramatic interior ceiling height without structural ridge beams.
Engineered with a central open rectangular box and heavy 2x10 floor chord, creating 12-to-16-foot wide bonus bedrooms or storage lofts within the roof envelope.
Why the two end trusses on a building differ from common trusses:
Gable end trusses have their top chord dropped by 1.5 inches (for 2x4 lookouts) or 3.5 inches (for 2x4 on-edge). This allows horizontal cantilever overhang boards (lookouts) to pass over the gable wall and support the fly rafter and bargeboard.
Instead of diagonal web triangulation, gable trusses feature vertical 2x4 studs spaced 16" or 24" on-center, providing flat structural backing for exterior plywood sheathing, housewrap, and siding.
In freezing winter weather, many homeowners notice ceiling drywall pulling away from interior partition walls:
Top chords are exposed to cold, damp winter air (high moisture), while bottom chords are buried under 16 inches of warm, dry attic insulation ($R-49$). The top chord expands while the bottom chord shrinks, causing the center of the truss to arch upward $1/2\text{"}$ to $3/4\text{"}$.
Never screw ceiling drywall within 12 inches of non-bearing interior partition walls. Use drywall corner clips or backing wood so ceiling drywall can flex smoothly with seasonal truss movement without cracking joint tape.
Over 80% of truss structural failures occur during erection before permanent lateral bracing is secured:
Continuous horizontal 2x4 runners nailed across compression web members prevent slender timber struts from buckling sideways under snow loads.
Permanent 45° diagonal braces nailed inside web bays transfer wind shear forces directly to building end walls, preventing domino-effect racking.
Hip roofs utilize 2-ply or 3-ply heavy girder trusses fitted with engineered steel joist hangers to support intersecting step-down hip sets and corner jack rafters.
Standard trusses compress attic insulation down to 3.5 inches at the eaves, causing heat loss, high energy bills, and destructive ice dams:
A Raised Heel truss elevates the top chord by 12 to 18 inches above the exterior wall plate. This allows full-thickness, uncompressed blown fiberglass or cellulose insulation to extend continuously over the top wall plate, eliminating cold corners and attic mold.
Building codes reward raised heel trusses with an insulation trade-off: if uncompressed insulation covers the entire top plate, the required ceiling insulation level can be reduced from R-49 down to R-38, saving material costs while delivering superior thermal performance.
Critical field protocols to protect factory-engineered trusses from out-of-plane damage:
Trusses spanning over 30 feet must never be lifted by a single apex choker sling. Always use a crane spreader bar with pick points at 1/3 and 2/3 chord spans to prevent lateral buckling.
Store trusses vertically braced or flat on level wood cribbing spaced no more than 8 to 10 feet apart. Never let trusses sag in direct contact with wet mud or puddles.
Standard trusses require at least 1.5 inches of bearing on wood framing plates (3.0" on masonry). Heavy multi-ply girder trusses require 3.5" to 5.5" of solid bearing support.
To calculate roof truss count: (1) Formula: Truss Count = (Building Length in Feet / On-Center Spacing in Feet) + 1. (2) Example for a 40-foot building with standard 24-inch (2-foot) spacing: (40 / 2) + 1 = 21 common trusses. (3) Gable Ends: Most homes require 2 additional dropped-top-chord Gable End Trusses (or converting the outer 2 trusses to gable ends) with vertical 16-inch studs to support exterior siding and lookout ladders. (4) Always verify if a hip roof is specified, which requires a specialized hip truss step-down package (girder truss, step-down trusses, corner jacks).
The primary differences lie in internal web geometry and span capacity: (1) Fink Truss ('W' Web): The most popular residential roof truss worldwide for spans up to 40 feet. It divides top chords into 4 equal panels with diagonal web struts resembling a 'W', optimizing lumber efficiency; (2) King Post Truss: The simplest truss with a single vertical post in tension at the apex. Ideal for short spans up to 24 feet (garages, sheds, porches); (3) Howe Truss: Features vertical tension members and diagonal compression struts sloping toward the center apex. Used for heavy industrial loads and long spans up to 60+ feet.
Prefabricated roof trusses typically cost between $4.50 and $12.00 per foot of span (or $80 to $450+ per individual truss delivered): (1) 24-ft Fink Truss (2x4 lumber): $85 to $140 each; (2) 30-ft Fink/Howe Truss: $130 to $220 each; (3) 40-ft Heavy-Duty Truss (2x6 top chord): $250 to $450 each; (4) Scissor Vaulted Truss: $150 to $300 each (15–25% premium); (5) Attic Room Truss (heavy 2x10 floor chord): $280 to $600+ each. A complete truss package for a 2,000 sq ft home (40-ft length, 24-ft span) averages $3,000 to $6,500.
A Scissor Truss features an inclined bottom chord that slopes upward from the exterior wall bearing plates to an interior apex, creating a soaring vaulted or cathedral ceiling inside while maintaining a steeper exterior roof pitch. The rule of thumb for scissor truss engineering is that the interior bottom slope is typically half (or 2/12 to 3/12 lower than) the exterior pitch. For example, an 8/12 exterior roof pitch is paired with a 4/12 interior ceiling vault pitch, allowing room for structural web triangulation and R-38+ attic insulation at the eaves.
Engineered light-frame wood roof trusses easily clear-span 30 to 60+ feet without requiring any interior load-bearing walls or intermediate support columns. The entire downward gravity load (dead weight + snow/wind loads) is transferred directly to the exterior wall top plates. This complete open clear-span capability allows modern open-concept floor plans where interior partition walls can be placed or moved anywhere without structural consequences.
NEVER cut, notch, drill, or remove any chord, web member, or metal gang-nail connector plate of an engineered roof truss without written approval from a licensed structural engineer. Truss members are calibrated precisely under high tension or compression; cutting even a single web member or 2x4 chord destroys the triangular load path and can cause catastrophic roof collapse. If attic storage or mechanical space is required, order custom-designed Attic Room Trusses with built-in rectangular walk-through frames.
